Conventional printed circuit boards (PCBs) consist of laminated non-conductive dielectric substrates that rely on drilled and plated through holes (PTHs) to form a connection between the opposite sides and/or inner layers of a board. Electroless plating is a well-known process for preparing metallic coatings on surfaces. Electroless plating of a dielectric surface requires the prior deposition of a catalyst. The most commonly used method to catalyze or activate laminated non-conductive dielectric substrate regions, prior to electroless plating, is to treat the board with an aqueous tin-palladium colloid in an acidic chloride medium. The colloid consists of a metallic palladium core surrounded by a stabilizing layer of tin(II) ions. A shell of [SnCl3]− complexes act as surface stabilizing groups to avoid agglomeration of colloids in suspension.
In the activation process, the palladium-based colloid is adsorbed onto an insulating substrate such as epoxy or polyimide to activate electroless copper deposition. Theoretically, for electroless metal deposition, the catalyst particles play roles as carriers in the path of transfer of electrons from reducing agent to metal ions in the plating bath. Although the performance of an electroless copper process is influenced by many factors such as composition of the deposition solution and choice of ligand, the activation step is the key factor for controlling the rate and mechanism of electroless deposition. Palladium/tin colloid has been commercially used as an activator for electroless metal deposition for decades, and its structure has been extensively studied. Yet, its sensitivity to air and high cost leave room for improvement or substitution.
While the colloidal palladium catalyst has given good service, it has many shortcomings which are becoming more and more pronounced as the quality of manufactured printed circuit boards increases. In recent years, along with the reduction in sizes and an increase in performance of electronic devices, the packaging density of electronic circuits has become higher and subsequently required to be defect free after electroless plating. As a result of greater demands on reliability alternative catalyst compositions are required. The stability of the colloidal palladium catalyst is also a concern. As mentioned above, the palladium/tin colloid is stabilized by a layer of tin(II) ions and its counter-ions can prevent palladium from aggregating. The tin(II) ions easily oxidize to tin(IV) and thus the colloid cannot maintain its colloidal structure. This oxidation is promoted by increases in temperature and agitation. If the tin(II) concentration is allowed to fall close to zero, then palladium particles can grow in size, agglomerate, and precipitate.
Considerable efforts have been made to find new and better catalysts. For example, because of the high cost of palladium, much of the effort has been directed toward the development of a non-palladium or bimetallic alternative catalysts. In the past, problems have included the fact that they are not sufficiently active or reliable enough for through-hole plating. Furthermore, these catalysts typically become progressively less active upon standing, and this change in activity renders such catalysts unreliable and impractical for commercial use. U.S. Pat. No. 4,248,632 discloses a non-palladium/tin catalyst for electroless plating. The catalyst includes a complex of a catalytic metal, such as palladium or alternative metals such as silver and gold, nitrogen containing ligands and acid radicals; however, an acid environment is critical for its activating performance. Acid environments typically cause undesired corrosion of metal cladding found on many dielectric substrates resulting in defective articles. This problem is very common in the manufacture of printed circuit boards where the boards are often heavily clad with copper, thus acid environments are highly undesirable in the industry.
Preferably metal clad dielectrics are electrolessly plated in alkaline environments but many non-palladium/tin catalysts are unstable and unreliable under such conditions. U.S. Pat. No. 5,503,877 discloses another non-palladium/tin catalyst which may be used in an acid as well as an alkaline environment. The catalyst is composed of a catalytic metal such as palladium, silver or gold, nitrogen containing ligands and solvent component; however, the catalyst must first be heated for prolonged periods of time to form an oligomer/polymer otherwise it is insufficiently active. Further, prolonged heating and subsequent cooling for packaging causes more cost due to increased manpower and facility charges in large scale catalyst preparation. Accordingly, there is still a need for a replacement catalyst for palladium/tin.